1. Field of the Invention
The present invention relates to reproducing apparatuses and methods, signal processing apparatuses and methods, and programs, and more particularly, to a reproducing apparatus and method, a signal processing apparatus and method, and a program suitable for use in reproducing channel data from a recording medium with high recording density using interpolated timing recovery (ITR).
2. Description of the Related Art
As the recording densities of recording media have increased in recent years, techniques for accurately reproducing channel signals from such higher density recording media have become necessary.
FIG. 1 illustrates an exemplary structure of a general reproducing apparatus of the related art for reproducing signals recorded on recording media, such as optical disks and magnetic disks.
Components of a reproducing apparatus 10 shown in FIG. 1 operate in synchronization with a channel clock generated by adjusting a reference clock on the basis of a reproduced-information signal read from a recording medium 11.
That is, in the reproducing apparatus 10, a reproduced-information signal (radio-frequency (RF) signal) read from the recording medium 11 by a disk drive 12 is waveform-equalized by an analog filter 20. The waveform-equalized signal is digitized (binarized) by an analog-to-digital (A/D) converter 21, and the binarized (digitized) signal is supplied to a phase-locked loop (PLL) circuit 22 and a waveform equalizer 27.
In the PLL circuit 22, a phase comparator 23 detects a phase difference between the binarized reproduced-information signal and an output signal of a voltage-controlled oscillator (VCO) 26, and a loop filter 24 generates a phase error signal corresponding to the phase difference detected. The phase error signal is converted by a digital-to-analog (D/A) converter 25 into an analog signal, which is then supplied to the VCO 26. The VCO 26 generates a channel clock which corresponds to the analog phase error signal and which is based on the reference clock and supplies the channel clock to a subsequent stage. In this manner, the channel clock in synchronization with the reproduced-information signal is generated.
The waveform equalizer 27 equalizes the waveform of the digitized reproduced-information signal, and the waveform-equalized signal is decoded by a Viterbi decoder 28. As a result, the decoded signal serving as channel data is generated.
The reproducing apparatus 10 is based on a structure combining waveform equalization to partial response and maximum likelihood detection (Viterbi detection). This structure is referred to as partial response maximum likelihood (PRML).
As is clear from the foregoing description, it is necessary for PRML represented by the reproducing apparatus 10 to use a channel clock generated by a PLL circuit in synchronization with a reproduced-information signal.
The reproducing apparatus 10 shown in FIG. 1 can reproduce channel data from the recording medium 11, such as a general optical disk of the related art. However, when reproducing channel data from a higher density recording medium, marks recorded on the recording medium are closer together and interfere with each other. Deterioration of the quality of a binary signal becomes a direct cause of difficulty for the PLL circuit 22 to correctly generate a channel clock in synchronization with a reproduced-information signal. As a result, it becomes difficult to reproduce channel data.
Assuming that marks recorded on a higher density recording medium interfere with each other, there is a demand for a structure that can perform timing recovery of a channel clock with higher accuracy than a PLL circuit of the related art.
FIG. 2 illustrates an exemplary structure of a reproducing apparatus for satisfying the foregoing necessity. A reproducing apparatus 30 shown in FIG. 2 is based on ITR. ITR is the technique of sampling a reproduced-information signal read from a recording medium 31 in synchronization with a fixed clock and performing timing recovery by use of phase interpolation. ITR is described in, for example, Japanese Unexamined Patent Application Publication No. 2003-317405.
In the reproducing apparatus 30, a predetermined fixed clock is generated by a frequency synthesizer 33 on the basis of a reference clock and is supplied to components of the reproducing apparatus 30. A reproduced-information signal (RF signal) read from the recording medium 31 by a media drive 32 is waveform-equalized by an analog filter 34. The waveform-equalized signal is sampled (digitized) by an A/D converter 35 in synchronization with the fixed clock, and the sampled (digitized) signal is supplied to a waveform equalizer 36. The waveform equalizer 36 equalizes the waveform of the digitized reproduced-information signal and supplies the waveform-equalized signal to a phase interpolator 37.
The phase interpolator 37 interpolates the phase of the digitized reproduced-information signal on the basis of a phase error signal sent as a feedback from an ITR-PLL circuit 40 and outputs the phase-interpolated signal to a Viterbi detector 38, a binary detector 39, and the ITR-PLL circuit 40.
The Viterbi detector 38 decodes the phase-interpolated signal and generates the decoded result serving as channel data.
The binary detector 39 binarizes the phase-interpolated signal, and the binary signal generated is supplied to the ITR-PLL circuit 40. The ITR-PLL circuit 40 generates a phase error signal corresponding to a phase difference between the phase-interpolated signal and the binary signal and outputs the phase error signal to the phase interpolator 37.
The reproducing apparatus 30 can reproduce channel data from the higher density recording medium 31. In the reproducing apparatus 30, however, since the waveform equalizer 36 is provided before the phase interpolator 37, the waveform-equalized signal is out of phase with the channel data generated as a result of Viterbi decoding. It is therefore difficult to adaptively change tap coefficients of a finite impulse response (FIR) filter constituting the waveform equalizer 36 using a simple method, such as the least mean square (LMS) algorithm or the like.